U.S. patent application number 14/347428 was filed with the patent office on 2014-11-06 for aqueous ink formulation containing metal-based nanoparticles for usage in micro contact printing.
This patent application is currently assigned to CLARIANT INTERNATIONAL AG. The applicant listed for this patent is BAYER INTELLECTUAL PROPERTY GMBH. Invention is credited to Venkataramanan Balasubramanian, Daniel Rudhardt, Frank Sicking, Deivaraj Theivanayagam Chairman.
Application Number | 20140329054 14/347428 |
Document ID | / |
Family ID | 54256903 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140329054 |
Kind Code |
A1 |
Theivanayagam Chairman; Deivaraj ;
et al. |
November 6, 2014 |
AQUEOUS INK FORMULATION CONTAINING METAL-BASED NANOPARTICLES FOR
USAGE IN MICRO CONTACT PRINTING
Abstract
The present invention relates to an aqueous formulation
particularly for generating electrically conductive and/or
reflective structures by microcontact printing, characterized in
that the formulation contains at least a) .gtoreq.15 to .ltoreq.55
parts by weight water, b) .gtoreq.10 to .ltoreq.50 parts by weight
alcohol, c) .gtoreq.15 to .ltoreq.45 parts by weight metal-based
nanoparticles, d) 0.5 to .ltoreq.10 parts by weight non-fluorinated
surfactant, and e) .gtoreq.0.5 to .ltoreq.10 parts by weight
fluorinated surfactant, wherein the above defined constituents a)
to e) summarize to a concentration of .ltoreq.100 parts by weight
in the formulation. The wetting behavior especially of hydrophobic
materials may significantly be improved. The present invention
further relates to a method of generating structures, particularly
being electrically conductive and/or reflective, on a substrate by
microcontact printing and a substrate comprising such a
structure.
Inventors: |
Theivanayagam Chairman;
Deivaraj; (Singapore, SG) ; Sicking; Frank;
(Overath, DE) ; Balasubramanian; Venkataramanan;
(Singapore, SG) ; Rudhardt; Daniel; (Koln,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAYER INTELLECTUAL PROPERTY GMBH |
Monheim |
|
DE |
|
|
Assignee: |
CLARIANT INTERNATIONAL AG
Muttenz
CH
|
Family ID: |
54256903 |
Appl. No.: |
14/347428 |
Filed: |
September 25, 2012 |
PCT Filed: |
September 25, 2012 |
PCT NO: |
PCT/EP2012/068835 |
371 Date: |
March 26, 2014 |
Current U.S.
Class: |
428/156 ;
101/487; 106/18.35; 106/31.13; 252/512; 252/514 |
Current CPC
Class: |
B41M 1/26 20130101; C09D
11/03 20130101; C09D 11/52 20130101; Y10T 428/24479 20150115; B41M
1/00 20130101; B41M 7/009 20130101 |
Class at
Publication: |
428/156 ;
252/512; 252/514; 106/31.13; 106/18.35; 101/487 |
International
Class: |
C09D 11/02 20060101
C09D011/02; B41M 1/26 20060101 B41M001/26; B41M 7/00 20060101
B41M007/00; C09D 11/00 20060101 C09D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2011 |
SG |
2011 07156-0 |
Claims
1. An aqueous formulation for generating electrically conductive
and/or reflective structures by microcontact printing, comprising
a) .gtoreq.15 to .ltoreq.55 parts by weight water, b) .gtoreq.10 to
.ltoreq.50 parts by weight alcohol, c) .gtoreq.15 to .ltoreq.45
parts by weight metal-based nanoparticles, d) .gtoreq.0.5 to
.ltoreq.10 parts by weight non-fluorinated surfactant, and e)
.gtoreq.0.5 to .ltoreq.10 parts by weight fluorinated surfactant,
wherein the above defined constituents a) to e) add up to a
concentration of .ltoreq.100 parts by weight in the
formulation.
2. A formulation according to claim 1, characterized in that the
metal-based nanoparticles comprise silver nanoparticles.
3. A formulation according to claim 1, characterized in that the
alcohol is ethanol, isopropanol, methanol, or a mixture
thereof.
4. A formulation according to claim 1, characterized in that the
metal-based nanoparticles comprise an average effective diameter of
.ltoreq.150 nm or a bimodal size distribution.
5. A formulation according to claim 1, characterized in that the
fluorinated surfactant comprises poly-(oxetane) polymers comprising
(--C.sub.2F.sub.5)-groups.
6. A formulation according to claim 1, characterized in that the
non-fluorinated surfactant comprises a siloxane.
7. A formulation according to any claim 1, characterized in that
the formulation further comprises at least one additive selected
from the group consisting of surfactants, pigments, defoamers,
light protecting agents, lighteners, whiteners, corrosion
inhibitors, antioxidants, algicides, plasticizers, softeners, and
thickeners.
8. A formulation according to claim 1, characterized in that the
formulation contains at least a) .gtoreq.31 to .ltoreq.42 parts by
weight water, b) .gtoreq.25 to .ltoreq.35 parts by weight alcohol,
c) .gtoreq.23.5 to .ltoreq.33.5 parts by weight metal-based
nanoparticles, d) .gtoreq.1 to .ltoreq.5 parts by weight
non-fluorinated surfactant, and e) .gtoreq.0.5 to .ltoreq.4.5 parts
by weight fluorinated surfactant, wherein the above defined
constituents a) to e) add up to a concentration of .ltoreq.100
parts by weight in the formulation.
9. A method of generating structures on a substrate by microcontact
printing, characterized by the steps of A) Providing a stamp; B)
Applying a formulation according to claim 1 to at least a part of
the surface of the stamp; C) Transferring the formulation from the
stamp to the substrate; and D) Optionally treating the formulation
transferred to the substrate with heat.
10. A method according to claim 9, characterized in that a stamp is
used which is at least partly formed from a hydrophobic
material.
11. A method according to claim 9, characterized in that a stamp is
used which is at least partly structured.
12. A substrate comprising a structure being particularly
electrically conductive and/or reflective and being obtainable by a
formulation according to claim 1 by microcontact printing.
13. A formulation according to claim 1, characterized in that the
metal-based nanoparticles comprise an average effective diameter of
.ltoreq.100 nm or a bimodal size distribution.
14. A formulation according to claim 8, characterized in that the
formulation contains at least a) .gtoreq.36 to .ltoreq.37 parts by
weight water, b) .gtoreq.29 to .ltoreq.31 parts by weight alcohol,
c) .gtoreq.28 to .ltoreq.29 parts by weight metal-based
nanoparticles, d) .gtoreq.2.5 to .ltoreq.3.5 parts by weight
non-fluorinated surfactant, and e) .gtoreq.1.5 to .ltoreq.3 parts
by weight fluorinated surfactant, wherein the above defined
constituents a) to e) add up to a concentration of .ltoreq.100
parts by weight in the formulation.
Description
[0001] The present invention relates to an aqueous formulation
containing metal-based nanoparticles, in particular silver,
especially for generating electrically conductive and/or optically
reflective structures particularly by microcontact printing, in
particular on flexible and/or transparent substrates using a stamp
made of poly-(dimethylsiloxane). The present invention further
relates to a method of generating structures, particularly being
electrically conductive and/or optically reflective, on a substrate
by microcontact printing using the above defined formulation. The
present invention further relates to a substrate comprising such a
structure.
[0002] Microcontact Printing (.mu.CP) is touted to be a simple and
a versatile printing process, that employs micro-patterned stamps,
for example made of poly-(dimethylsiloxane) (PDMS) in order to
print micro scale features onto various substrates. The substrates
may include those being curved and having a large area.
Microcontact printing can be used to print, inter alia, self
assembled monolayers (SAM's), polymers, dendrimers, catalysts or
biomolecules such as proteins, liposomes, etc. on substrates of
choice. However, there are very few reports on printing
nanoparticles using microcontact printing. For instance, printing
of titanium dioxide (TiO.sub.2) nanoparticles or quantum dots using
microcontact printing have been reported.
[0003] By using stamps made of poly-(dimethylsiloxane),
furthermore, this material being a non-polar elastomer with a
hydrophobic surface presents some challenges due to its low surface
energy, thus resulting in poor wetting of polar ink systems, such
as aqueous based ink systems. Therefore, methods are known to
modify the surface of the poly-(dimethylsiloxane) stamps in order
to increase the surface energy and furthermore to improve the
wetting behavior of polar ink systems. These modifying procedures
are typically multistep processes involving plasma treatment and
surface grafting of polar moieties on the poly-(dimethylsiloxane)
surface. However, the surface after the modification route often
presents limited success as the wettability appears to be getting
poor with repeated usage and/or storage.
[0004] An alternative would be to microcontact print metallic
colloids directly onto substrates of choice. However, there appears
to be not much information on printing metallic nanoparticles using
microcontact printing. For instance, microcontact printing of Pd/Sn
or Pd nanoparticles on poly-(dimethylsiloxane) stamps was reported
as seeds for a further electroless deposition of NiB or copper. It
has to be noted that the Pd/Sn nanoparticles are only used as seeds
layers and this could as well be a random deposition of
nanoparticles and not really dense structures. It must also be
highlighted that these seed nanoparticles are often dispersed in
organic solvents like toluene or hexane.
[0005] Most of the known printing of metallic conducting pattern
was performed by passivating gold surfaces by a self assembled
monolayer of a suitable thiol followed by
electrodeposition/electroless deposition of a metallic species on
the unpassivated sections of the substrates and finally a wet
etching to remove the uncoated areas of the gold surface. It is
obvious that this type of patterning is process intensive and
cumbersome.
[0006] Known from US 2009/0191355 A1 is a method of forming a layer
of particulate on a substrate, and in particular, a method of
forming a thin layer of nanometer sized particulate on a substrate
for use in microfabrication of components and devices. The method
according to this document generally comprises the steps of
providing an elastomeric stamp having a relief structure; applying
a composition comprising particulate and a dispersing agent to the
relief structure; selectively transferring the composition from the
relief structure to the substrate to form the pattern; treating the
composition with charged gas to remove the dispersing agent; and
induction heating to form functional connection of the particulate.
The ink used for performing this method may contain conductive
materials such as silver and in particular silver nanoparticles, a
binder, and methanol as solvent.
[0007] Document WO 2009/052120 A1 describes a method of
microfabrication and nanofabrication of electrical and mechanical
structures at the micron and submicron scale, for example by
microcontact printing. This method uses a formulation comprising a
plurality of metallic nanoparticles, such as silver nanoparticles,
suspended in a carrier, wherein the carrier comprises water and at
least one organic solvent miscible with water. The organic solvent
being miscible with water may be an alcohol, such as terpene
alcohol or a polyol, such as glycol or glycerol, or a long chain
alcohol, such as octanol or decanol. Furthermore, the formulation
known from this document may comprise additives such as surfactants
or dispersants.
[0008] However, with respect to the above defined prior art, the
swelling and wetting behavior has further potential to be improved
especially in absence of surface modification of the
poly-(dimethylsiloxane) stamp. In detail, a plasma or UV/ozone
surface treatment for wetting of the stamp such as the stamp made
of poly-(dimethylsiloxane) should be avoided.
[0009] Consequently, there is the need for further ink formulations
improving the printability and particularly the wetting behavior
especially of a hydrophobic material such as a
poly-(dimethylsiloxane) stamp without pretreating the stamp. It is
thus the object of the present invention to provide an aqueous
formulation being usable as ink for microcontact printing which has
an improved wetting behavior of a hydrophobic material such as of a
stamp made of poly-(dimethylsiloxane).
[0010] The present invention relates to an aqueous formulation
particularly for generating electrically conductive and/or
reflective structures by microcontact printing, characterized in
that the formulation contains at least [0011] a) .gtoreq.15 to
.ltoreq.55 parts by weight water, [0012] b) .gtoreq.10 to
.ltoreq.50 parts by weight alcohol, [0013] c) .gtoreq.15 to
.ltoreq.45 parts by weight metal-based nanoparticles, [0014] d)
.gtoreq.0.5 to .ltoreq.10 parts by weight non-fluorinated
surfactant, and [0015] e) .gtoreq.0.5 to .ltoreq.10 parts by weight
fluorinated surfactant, wherein the above defined constituents a)
to e) summarize to a concentration of .ltoreq.100 parts by weight
in the formulation.
[0016] The term "metal-based" nanoparticles in the sense of the
present invention shall particularly mean any nanoparticles which
comprise a metal as such or a compound being at least partly formed
from a metal compound, such as an alloy, metal oxides or the like.
As an example for metal oxides, titanium oxide (TiO.sub.2) or
indium tin oxide (ITO) may be referred to in an exemplary manner
only. Consequently, at any passage metal-based nanoparticles are
cited, these particles may be metals, alloys, or metal compounds
such as metal oxides. Apart from that, the term "nanoparticles" in
the sense of the present invention may exemplarily mean particles
having a maximum diameter in a range of .ltoreq.250 nm, for example
lying in the range of .gtoreq.1 nm to .ltoreq.250 nm
[0017] The term "structure" in the sense of the present invention
shall particularly mean any kind of material being applied to the
surface of a substrate. In detail, the term structure comprises a
layer being applied to a whole or an expanded region of a surface
area, such as a large region coating, or a defined pattern being
applied just to defined regions onto the surface of the
substrate.
[0018] The term "microcontact printing" according to the present
invention shall particularly mean a process being generally known
in the art. This process comprises the steps of applying a
formulation, or an ink, respectively, onto a surface or at least
onto a part of the latter of a stamp, which may be structured or
not. The ink being applied to the stamp is in turn transferred to a
suited substrate in order to apply a structure onto the latter.
[0019] A formulation according to the present invention overcomes
the problems of wetting without having to subject a hydrophobic
surface, such as a poly-(dimethylsiloxane) surface, to a lengthy
and cumbersome surface modification procedure. Apart from that, the
formulation according to the invention does not swell stamps made
of a hydrophobic material, such as poly-(dimethylsiloxane).
[0020] Without being bound to the theory it is believed that the
positive effects and advantages being provided by an aqueous
formulation particularly for generating electrically conductive
and/or reflective coatings according to the invention are obtained
by synergistic effects of the respective components. Particularly,
it is believed that the advantages are obtained by the components
being present in the formulation according to the invention in a
respective concentration range.
[0021] Next to the components defined above, the ink formulation
according to the invention may comprise further constituents
without leaving the scope of the invention as such. For example,
the formulation may comprise at least one additive in order to
improve one or more of the properties of the formulation or to
adapt them to the special use. The further one or more additives
being potentially part of the ink formulation according to the
invention may be selected from the group comprising or consisting
of surfactants, pigments, defoamers, light protecting agents,
lighteners, wighteners, corrosion inhibitors, antioxidants,
algicides, plasticizers, softeners, and/or thickeners, the list not
being strictly final.
[0022] The components being present in the ink formulation
according to the present invention are particularly chosen in view
of the wetting behavior on a stamp with regard to microcontact
printing in a method of generating electrically conductive and/or
reflective structures. Consequently, the constituents being present
in the ink formulation according to the present invention are
particularly chosen in view of the wetting behavior on hydrophobic
materials, such as poly-(dimethylsiloxane).
[0023] Within the in formulation according to the invention, the
metal-based nanoparticles essentially serve as main ingredient of
the particularly electrically conductive and/or reflective
structure to be generated on the substrate of choice. They may be
present in the formulation according to the invention in a
concentration in the range of .gtoreq.15 to .ltoreq.45 parts by
weight. It is clear for one skilled in the art that either the same
kind of nanoparticles may be present in the formulation according
to the invention, or different kinds of nanoparticles may be
present in the formulation according to the invention without
leaving the invention as such.
[0024] The water being present may serve as solvent for dispersing
the metal-based nanoparticles. It may be present in a concentration
in the range of .gtoreq.15 to .ltoreq.55 parts by weight.
[0025] Within the ink formulation according to the present
invention, the alcohol may be present in a concentration in the
range of .gtoreq.10 to .ltoreq.50 parts by weight and may serve as
co-solvent. Additionally, it may take the role as wetting
agent.
[0026] With respect to the non-fluorinated surfactant, the latter
may especially serve as agent for reducing the surface tension of
the ink, so that the wetting behavior of the stamp may be further
improved. It may be present in a concentration in the range of
.gtoreq.0.5 to .ltoreq.10 parts by weight.
[0027] With respect to the fluorinated surfactant, the latter may
especially serve as leveling agent and/or agent for reducing the
surface tension of the ink, providing positive aspects with respect
to the wetting behavior. It may be present in a concentration in
the range of .gtoreq.0.5 to .ltoreq.10 parts by weight in the
formulation.
[0028] The ink formulation according to the invention may provide
superb wetting behavior, which is especially advantageous with
respect to microcontact printing. In detail, by having a good
wetting behavior, the stamp, especially being formed of a
hydrophobic material, such as poly-(dimethylsiloxane), may be
wettened and thus provided with the ink formulation in a defined
manner. Consequently, especially in case the stamp is structured in
order to generate a defined structure on a substrate, a good
wetting behavior is advantageous in order to generate the desired
structure by transferring the ink to the substrate. A structured
stamp may thereby particularly mean a stamp having at least one
surface being provided with a structure. The structure, in order to
be suitable for microcontact printing, may particularly have
protruding and recessed portions forming the required structure.
The structure as such may be adjusted to the desired application.
It may thus comprise defined areas, lines or spots, for example. By
addressing the problems known in the art with respect to wetting,
it may be assured that the desired geometry and form of the
structure to be applied, as defined by the stamp or the stamp
structure, may accurately be transferred to a substrate of
choice.
[0029] It is thereby not necessary to modify the surface of the
stamp by chemically or physically processing the latter. The
wettability may instead be improved by the ink formulation as such.
Consequently, a printing process may be performed without a further
step resulting in a highly efficient and cost saving process.
[0030] The dimensions and/or structures are furthermore just
dependent from the stamp, or its structure, respectively, being
used and the pressure applied to the latter. Consequently, by using
an ink formulation according to the present invention, it is
possible to print conducting and/or reflecting metal structures,
for example, in various dimensions.
[0031] It is also possible to print semi-transparent grid
structures, for example with sheet resistances less than 5
.OMEGA./sq., especially if appropriate sintering conditions are
chosen after transferring the formulation to the substrate. With
respect to the electrically conductive structures being formed on
the substrate of choice, such as conducting paths, or large area
coatings, they may preferably be temperature resistant, for example
at least for a short period of time up to 4000.degree. C., as well
as mechanically flexible. The ink formulation according to the
invention may furthermore be suitable for generating structures, or
lines, respectively, having a width of 100 .mu.m or less (up to 20
.mu.m or even lower). This may especially be advantageous with
respect to small dimensioned substrates. Apart from that, the
applicability of the ink formulation according to the present
invention is especially broad.
[0032] Furthermore, the ink formulation according to the present
invention is especially cost-saving to prepare and to use and,
additionally, may provide a superb shelf life. Furthermore, due to
the fact that the desired structures may appropriately be
generated, the degree of substrate being falsely coated and thus
not being usable for the desired application may be reduced up to a
minimum Consequently, the ink formulation according to the present
invention may provide a high efficiency at its use.
[0033] Besides, the ink formulation according to the present
invention prevents the stamp used for printing from swelling and/or
shrinking. In detail, shrinking is a process due to which the stamp
changes its dimensions leading to the dimensions of the structure
being present on the printing surface of the stamp as well being
changed. Consequently, the printed structure will not have the
dimensions desired in case the stamp shrinks. Additionally, most
organic solvents lead to a swelling process of a stamp particularly
formed from poly-(dimethylsiloxane), as well having negative
effects to the stamp and thus to the printing results. These above
named disadvantaged may be prevented by using an aqueous based
formulation according to the present invention.
[0034] Even if the ink formulation according to the present
invention is particularly suitable for microcontact printing and in
more detail for microcontact printing using a hydrophobic stamp, it
is especially suitable for any kind of printing technology
employing a hydrophobic material to transfer a structure, or a
pattern, respectively, to a substrate of choice.
[0035] It is the benefit of the present inventors to have found out
that the object of the present invention is surprisingly solved by
a suitable choice of an ink formulation having defined constituents
particularly in defined concentration ranges. The effect according
to the invention is provided, without being bound to a specific
theory, particularly by synergistic effects of solvents,
co-solvents and surfactants and potentially further additives
especially in defined concentration ratios.
[0036] According to an embodiment, the metal-based nanoparticles
comprise silver nanoparticles. Preferably, all metal nanoparticles
are silver nanoparticles. The silver nanoparticles may preferably
be used, or introduced into the formulation, respectively, in the
form of a silver nanoparticle sol (Ag sol). The silver nanoparticle
sol may be treated and thus particularly purified and concentrated
by using membrane filtration comprising a filter element with a
level of filtering of 100.00 dalton at most, for example. The
silver nanoparticle sol preferably comprises a dispersing agent,
which may be formed from a block-copolyether comprising styrene
blocks, with 62 parts by weight C.sub.2-polyether, 23 parts by
weight C.sub.3-polyether, and 15 parts by weight polystyrene, with
respect to the dried dispersing agent, with a relation of the
length of the blocks C.sub.2 polyether to C.sub.3 polyether of 7:2
units (for example Disperbyk 190, purchasable by BYK-Chemie,
Wesel). By way of the dispersing agent, which may serve as capping
agent, the silver nanoparticles are stabilized appropriately.
Consequently, agglomeration of the silver nanoparticles may be
prevented.
[0037] According to a further embodiment the alcohol may be
ethanol, isopropanol, methanol or a mixture comprising at least one
of the afore-mentioned compounds. Particularly by using methanol,
the wettability properties of the ink according to the invention
were found to be especially improved. Furthermore, methanol has a
preferred evaporation rate providing a very short drying time of
the silicone stamp provided with the ink formulation, or the
substrate provided with the structure. Apart from that, methanol is
cost-saving to use and is furthermore non problematic with respect
to its handling conditions.
[0038] According to a further embodiment the metal-based
nanoparticles comprise an average effective diameter of .ltoreq.150
nm, particularly of .ltoreq.100 nm, for example of .gtoreq.40 nm to
.ltoreq.80 nm, and/or a bimodal size distribution. The
determination of the size, and the size distribution, respectively,
via laser correlation spectroscopy is known in the art and
described, for example, in T. Allen, Particle Size Measurements,
Bd. L, Kluver Academic Publishers, 1999. In case silver
nanoparticles are used, they may preferably be used, or introduced
into the formulation, respectively, in the form of a silver
nanoparticle sol (Ag sol).
[0039] The bimodal size distribution may especially be preferred
with respect to electrically conductive structures such as patterns
or coatings, having a low content of metal-based nanoparticles such
as metal nanoparticles. It is believed that this effect is due to a
filling of the occurring gusset volumes between larger particles by
smaller particles. This results in large and continuous contact
areas to be formed especially during thermal treatment of the ink
formulation applied to the substrate. Consequently, the ink
formulation according to the invention reaches, with low content of
metal-based particles, the same electrical conductivity compared to
formulations having a higher content of nanoparticles with
monodispers size distributions of the nanoparticles and a
comparable effective diameter, or even higher electrical
conductivities compared to monodispers size distributions
comprising a comparable amount of metal-based nanoparticles having
the same effective diameter.
[0040] Due to the small effective diameter of the metal-based
nanoparticles, structures having a very small width may
additionally be achieved, which is especially preferred for defined
patterns and/or for compact substrates. Apart from that, a
structure may be achieved with a high contrast.
[0041] According to a further embodiment the fluorinated surfactant
comprises poly-(oxetane) polymers comprising
(--C.sub.2F.sub.5)-groups. These kinds of surfactants provide a
plurality of advantageous properties. In detail, these surfactants
have been found not to bioaccumulate. There is thus very low
environmental impact because of which these surfactants are
environmentally preferred even if being fluorosurfactants. Apart
from that, the foam being generated may be reduced due to a reduced
air entrapment because of which these surfactants lead to an
improved wetting behavior and thus printing result. Furthermore,
these surfactants are clear and uniform because of which they do
not deteriorate the desired appearance of the ink formulation.
Generally, flow, leveling, and surface appearance may be improved
by using the surfactants like described above. The surfactants used
according to this embodiment are purchasable under the names
PolyFox PF-136A, PF-156A, and PF-151N from the company Omnova, for
example.
[0042] According to a still further embodiment the non-fluorinated
surfactant comprises a siloxane, in particular a polyalkyleneoxide
modified heptamethyltrisiloxane. These kinds of surfactants are
especially preferred wetting agents reducing the surface tension of
the ink formulation according to this embodiment in an especially
preferred manner. Particularly by using these kinds of non ionic
surfactants, the wetting behavior and thus the distribution of the
ink formulation, for example on a stamp and essentially independent
from the stamp material, may be improved. For example, according to
this embodiment, the non-fluorinated surfactant may be the one
being purchasable under its name Silwet L77 from the company GE
Silicones.
[0043] According to a still further embodiment the formulation
contains at least [0044] a) .gtoreq.31 to .ltoreq.42, in particular
.gtoreq.36 to .ltoreq.37 parts by weight water, [0045] b)
.gtoreq.25 to .ltoreq.35, in particular .gtoreq.29 to .ltoreq.31
parts by weight alcohol, [0046] c) .gtoreq.23.5 to .ltoreq.33.5, in
particular .gtoreq.28 to .ltoreq.29 parts by weight metal-based
nanoparticles, [0047] d) .gtoreq.1 to .ltoreq.5 in particular
.gtoreq.2.5 to .ltoreq.3.5 parts by weight non-fluorinated
surfactant, and [0048] e) .gtoreq.0.5 to .ltoreq.4.5, in particular
.gtoreq.1.5 to .ltoreq.3 parts by weight fluorinated surfactant,
wherein the above defined constituents a) to e) summarize to a
concentration of .ltoreq.100 parts by weight in the
formulation.
[0049] According to this embodiment, especially good results
particularly with respect to microcontact printing may be achieved.
In detail, the wetting behavior of hydrophobic substrates, such as
poly-(dimethylsiloxane) is especially improved leading to exact and
defined structures to be formed even in case the structures have
very small dimensions.
[0050] It may be seen that the content of solvent and co solvent
may be comparable. For example, the relation between water and
alcohol may be 1/1.
[0051] Additionally, the amount of surfactants may be realized to a
minor amount so that the composition essentially comprises
metal-based nanoparticles, such as silver nanoparticles, water, and
alcohol, such as methanol.
[0052] The present invention further relates to a method of
generating structures, particularly being electrically conductive
and/or reflective, on a substrate by microcontact printing,
characterized by the steps of [0053] A) Providing a stamp; [0054]
B) Applying a formulation according to the invention to at least a
part of the surface of the stamp; [0055] C) Transferring the
formulation from the stamp to the substrate; and [0056] D)
Optionally treating the formulation transferred to the substrate
with heat.
[0057] The method according to the invention thus defines a micro
printing process using the ink formulation according to the
invention.
[0058] According to step A), a stamp is provided. The stamp may be
formed from a suitable material, such as a hydrophobic material.
However, the advantages such as the wetting behavior being obtained
by using the formulation according to the invention may generally
as well be achieved by using hydrophilic stamps. Particularly,
however, the stamp may at least partly be formed from
poly-(dimethylsiloxane). Additionally, the stamp may be structured
and may thus comprise the structure which is to be printed on the
substrate. In other words, the stamp may at least partly be
structured particularly on that surface being used for printing
purposes. Consequently, the exact form of the stamp or at least of
one surface of the latter is dependent on the desired printing
image. As a result, the stamp may comprise one large flat surface
in case a large coating is to be applied to the substrate.
Furthermore, the stamp may comprise a defined pattern in case such
a pattern is to be applied to the surface of the substrate. For
example, the pattern may correspond to a pattern of conducting
lines being required for electrical compounds, and may thus
comprise relief patterns. This may be realized, for example, by
respective protruding and recessed portions on the surface of the
stamp, like it is known from microcontact printing as such.
[0059] According to step B), a formulation according to the
invention is applied onto at least a part of the surface of the
stamp. In detail, the ink formulation is applied to the printing
surface of the stamp and thus to that surface being used for
printing purposes. Consequently, the formulation is applied to the
surface comprising the desired structure or pattern, respectively
by any known and appropriate technique thereby wetting the latter.
For example, the ink formulation may be applied to the printing
surface of the stamp by immersing the latter at least partly into
the formulation or by spraying the formulation onto the stamp, for
example. After having wetted the surface of the stamp with the
formulation, the excess formulation may be removed, or wicked,
respectively, from the stamp, for example by using a wire bar.
[0060] According to step C), the formulation is transferred from
the stamp to the substrate in order to generate the structure on
the substrate. In other words, the surface of the stamp being
treated with the formulation, i.e. the printing surface, is brought
into physical contact with the desired surface of the substrate to
be printed. In case the printing surface of the stamp comprises
protruding and recessed portions, for example, the ink may wet both
portions during step B). However, only the formulation being
present on the protruding portions will be transferred to the
substrate so that the desired structure is applied to the surface
of the substrate. Especially in case the stamp is elastic this step
may be improved.
[0061] The substrate to which the formulation is transferred may,
for example, be such a substrate being electrically insulating or
having only a limited electrical conductivity, for example formed
from a flexible material. As an example, the substrate may be
formed from glass or plastics, such as a glass plate or a plastic
foil, or it may be a polymer, such as a polymer film, or a silicium
wafer, for example.
[0062] Additionally, according to step D) a step of treating the
formulation transferred to the substrate with heat may follow.
During this step, the formulation may be sintered in order to
achieve a coating having especially improved properties, i.e.
particularly with respect to being electrically conductive and/or
being optically reflective. Additionally, the solvents and/or
liquids being present in the formulation may be removed. Step D)
may be performed under mild conditions. For example, temperatures
of more than 40.degree. C. may be used. However, preferred
temperatures may lie in the range of .gtoreq.150.degree. C. to
.ltoreq.500.degree. C., particularly in the range of
.gtoreq.300.degree. C. to .ltoreq.400.degree. C., for example at
350.degree. C. The temperature range may preferably be chosen in
dependence of the substrate and may thus be maintained below the
melting temperature or softening point of the substrate. It is thus
possible to achieve electrically conductive and/or optically
reflective structures on temperature sensible substrates. Step D)
may for example be performed by laser sintering, microwave
sintering, or by low temperature sintering. However, step D) is in
some cases not strictly necessary and is thus not mandatory, but
optional.
[0063] With respect to the duration of step D), the temperature
treatment may be performed, for example, for a period of .gtoreq.1
minute to .ltoreq.24 hours. Preferred durations lie in the range of
.gtoreq.5 minutes to .ltoreq.120 minutes, for example.
[0064] With respect to further advantages of the method according
to the invention it is referred to the above remarks with respect
to the inventive formulation.
[0065] The present invention further relates to a substrate
comprising a structure being particularly electrically conductive
and/or reflective and being obtainable by a formulation according
to the invention, particularly by microcontact printing.
[0066] Electrically conductive structures according to the present
invention are particularly patterns and/or coatings having an
electrical conductivity being suitable for conducting paths.
Accordingly, electrically conductive structures are particularly
and exemplarily those having a conductivity of more than 10
.mu.S/cm.
[0067] Due to a usage of the formulation according to the invention
especially in combination with microcontact printing in order to
obtain the substrate according to the invention, the electrically
conductive and/or optically reflective structures may have any
desired shape and geometry. The structures may comprise lines, or
patterns, respectively, having a width in the range of less than
100 .mu.m, for example up to 20 .mu.m.
[0068] The particularly electrically conductive and/or optically
reflective structures on the substrate may be flexible so that by
bending the substrate, the conductivity, for example, is
maintained. Additionally, the substrate may be transparent.
[0069] The substrate may preferably at least partly be formed from
a material being selected from the group consisting of glass,
polyimide (PI), polycarbonate (PC), polyethylenterephtalate (PET).
These materials provide suitable surface behaviors with respect to
printing and may easily be functionalized. However, the list of
substrate materials is not limited to the above named examples.
[0070] The combination of mechanical properties such as stability
or flexibility, optical properties such as transparency or
reflectivity and/or the electrical properties such as electrical
conductivity especially with respect to transparent plastics lead
to a broad range of applications of a substrate having a structure
like defined above. Especially preferred applications comprise in a
non limiting manner windows such as for vehicles, devices or
buildings being coupled with electrical applications (heating,
discharging electrical charges, shielding of electromagnetic
waves), or solar cells especially with respect to their sides
facing the sun. Thereby, the degree of freedom with respect to
design is nearly unlimited furthermore increasing the range of
applications.
[0071] With respect to further advantages of the substrate
according to the invention it is referred to the above remarks with
respect to the inventive formulation as well as the inventive
method.
[0072] The present invention is subsequently described with regard
to embodiments and with respect to the figures, without being
limited to the following description.
[0073] FIG. 1a shows a microscope image of a polycarbonate foil
being used for obtaining a stamp for performing the method
according to the invention;
[0074] FIG. 1b shows a microscope image of a further polycarbonate
foil being used for obtaining a stamp for performing the method
according to the invention;
[0075] FIG. 2a shows a microscope image of a part of a stamp being
obtained from the foil according to FIG. 1a;
[0076] FIG. 2b shows a microscope image of a part of a stamp being
obtained from the foil according to FIG. 1b;
[0077] FIG. 3a shows a microscope image of an embodiment of a
printed structure generated by the method according to the
invention;
[0078] FIG. 3b shows a microscope image of a further embodiment of
a printed structure generated by the method according to the
invention;
[0079] FIG. 3c shows a microscope image of a further embodiment of
a printed structure generated by the method according to the
invention; and
[0080] FIG. 4 shows a microscope image of a further embodiment of a
printed structure generated by the method according to the
invention.
EXAMPLE
[0081] The following ink formulation according to the present
invention was used (table 1), wherein L77 represents the
non-fluorinated surfactant "Silwet L77", purchasable under its name
by the company GE silicones, "Polyfox 156" represents the
fluorinated surfactant, purchasable by its name by the company
Omnova, methanol represents conventional methanol, Ag sol
represents silver nanoparticles stabilized by Disperbyk 190,
purchasable by BYK-Chemie, and DI water represents deionized
water.
TABLE-US-00001 TABLE 1 Weight % Conc. Of Raw % Conc. In Serial No.
Raw materials (g) Material Ink 1 L77 1.5 100.00 2.99 2 Polyfox 156
11.20 10.00 2.24 3 Methanol 14.95 100.00 29.90 4 Ag Sol 22.40 63.63
28.5 5 DI Water 36.37
[0082] The above defined formulation was used for microcontact
printing. Consequently, with regard to the method according to the
present invention, firstly, a stamp has to be provided. As stamp
material, poly-(dimethylsiloxane) was chosen.
[0083] The respective stamp was prepared using Sylgard 184,
purchasable by the company Dow Corning. This 2-part silicone
elastomer can be cured at room temperature, as well as up to
temperatures of 150.degree. C. The silicone elastomer base and the
curing agent of the Sylgard 184 mixture were mixed in the ratio
10:1. The mixture was then transferred into a Thinky biaxial mixer
for 90 sec at 2000 rpm and subsequently for 60 sec at 2200 rpm for
defoaming. The slurry obtained was added into a beaker containing
structured polycarbonate foils from Dr. Pudliner.
[0084] Digital images showing the structures of different
polycarbonate foils used for generating the patterned stamps are
shown in FIGS. 1a and 1b. From FIG. 1a it can be seen that the foil
comprises protuding regions (4 and 5) and recessed regions (1 to
3). The protruding regions all have thicknesses in the range of
.gtoreq.20 .mu.m to .ltoreq.25 .mu.m, whereas the recessed regions
have dimensions in the range of .gtoreq.18 .mu.m to .ltoreq.21
.mu.m. The foil according to FIG. 1b is comparable to the foil
according to 1a even though the recesses are deeper with regard to
the whole thickness of the foil. The structure being present on the
foils corresponds to the structure of the stamp and thus of the
structure to be printed onto the substrate.
[0085] After having added the slurry into a beaker containing
structured polycarbonate foils like described above, the beaker was
then kept in a vented oven and the poly-(dimethylsiloxane) slurry
was cured at 125.degree. C. for 1 h. The beaker was then taken out
allowed to cool down to room temperature and was broken to retrieve
the poly-(dimethylsiloxane) stamp to be used as stamp in
microcontact printing.
[0086] The stamps being obtained are shown in FIG. 2, wherein the
stamp shown in FIG. 2a is obtained from the foil according to FIG.
1a whereas the stamp being shown in FIG. 2b is obtained from the
foil being shown in FIG. 1b.
[0087] Having formed the stamp, the latter may be used for
microcontact printing using the formulation shown in table 1. The
microcontact printing process was performed as discussed below. The
poly-(dimethylsiloxane) stamp was wetted with the silver
nanoparticle ink of the formulation according to the invention. The
excess ink was then wicked by using a 10 .mu.m wire bar. This step
may be especially preferred as it helps removing the excess ink
from the stamp and thereby allows a good transfer of the pattern
from the stamp to the substrate of choice. It is also possible to
increase or even eliminate the aforementioned wicking step by
increasing the viscosity of the ink system, so that only the
"hills" and thus the protruding portions of the stamp are wettet,
or coated, respectively, and not the "valleys", or the recessed
portions.
[0088] After having wetted the stamp or its structure,
respectively, with the formulation, the structure is brought into
physical contact with the substrate, in this case being a glass
substrate, in order to generate an electrically conductive and/or
optically reflective structure onto the surface of the substrate.
The structure according to this example comprised a plurality of
thin lines. In order to improve or to generate the electrical
conductivity, the silver lines obtained could be sintered in an
oven under preferred conditions, such as a temperature range of
.gtoreq.150.degree. C. to .ltoreq.500.degree. C. for a time range
of .gtoreq.1 minute to .ltoreq.2 hours to make them especially
electrically conductive and to remove the solvent, or liquids,
respectively. Digital images of the lines thus printed could be
seen by microscope images below in FIGS. 3a, 3b and 3c. Different
line thicknesses could be obtained by choosing
poly-(dimethylsiloxane) stamps with different dimensions or by
adjusting the pressure during the printing process in an
appropriate manner.
[0089] In detail, FIG. 3a shows a pattern of silver lines
comprising a line width of approximately 17 .mu.m and spacings
there between of approximately 30 .mu.m. FIG. 3b shows a pattern of
silver lines comprising a line width of approximately 40 .mu.m and
spacings there between of approximately 95 .mu.m. FIG. 3c shows a
pattern of silver lines comprising a line width of approximately 10
.mu.m and spacings there between of approximately 40 .mu.m.
[0090] It was also possible to print semi-transparent grid patterns
by performing two consecutive printings, with orientations
perpendicular to each other. For example, after the first print of
the silver nanoparticle ink the glass substrate with the pattern
was sintered at 350.degree. C. for 10 min. After which, the
substrate was cooled and a second print with an orientation
perpendicular to the first print was made. The substrate with the
print was also sintered at 350.degree. C. for another 10 min. This
process resulted in a semi-transparent conductive grid pattern. The
microscope images are given in FIG. 4 showing microscope images of
the printed grid lines. The grid lines comprise a 12 .mu.m grid
with a 25 .mu.m spacing. The sheet resistance was measured to be
0.5 .OMEGA./sq. The lines printed show a thickness of about 250
nm.
[0091] As shown above, the ink formulation according to the
invention allows printing very thin lines, or patterns,
respectively. Consequently, the pattern being formed by the stamp
is appropriately transferred to the substrate. This shows a very
well wetting behavior and furthermore very well printing
results.
[0092] Additionally, the improved wetting behavior of the
formulation according to the invention could be seen by applying it
to a non structured and thus plane poly-(dimethylsiloxane) surface.
In detail, when having applied the formulation defined in table 1
onto such a poly-(dimethylsiloxane) surface, a contact angle of
33.3.degree. (with a standard deviation of 0.5.degree.) on the
surface was obtained. This shows that even hydrophobic surfaces may
be applied with the ink formulation according to the invention very
well leading to an improved wetting behavior.
[0093] While the invention has been illustrated and described in
detail in the drawings and foregoing description, such illustration
and description are to be considered illustrative or exemplary and
not restrictive, the invention is not limited to the disclosed
embodiments. Other variations to the disclosed embodiments can be
understood and effected by those skilled in the art in practicing
the claimed invention, from a study of the drawings, the
disclosure, and the appended claims. In the claims, the word
"comprising" does not exclude other elements or steps, and the
indefinite article "a" or "an" does not exclude a plurality. The
mere fact that certain measures are recited in mutually different
dependent claims does not indicate that a combination of these
measures cannot be used to advantage. Any reference signs in the
claims should not be construed as limiting the scope.
* * * * *